In dewatering pumping installations, the pumps are operated until the vessel or sump is empty or nearly empty and then they are turned off until the vessel or reservoir becomes full once more. In reservoir filling installations the converse is true. Such pump installations require level sensors to determine the maximum and minimum levels for activating and deactivating the pumps. Where the vessel or reservoir collects highly corrosive industrial wastes, raw sewage, or septic-tank effluent, mechanical and electrically operated level sensors become corroded or rendered inoperative by clogging from sticks, rags, abrasives, hair, etc. In some installations, the accumulating liquids may create gases which are highly inflammable and electrically operated sensors pose a danger of explosion or fire. Level sensors in such corrosive or sensor-clogging environments are often difficult to service and require frequent attention.
Pumps for handling sewage, sump, bilge and industrial waste waters and liquids are generally centrifugal units. Where the liquid is highly inflammable, or the material such as raw sewage contains a variety of solids -- sticks, rags, rocks, hair, and other clogging material, pneumatic ejectors have frequently been preferred to electrically driven centrifugal pumps. Such pneumatic ejectors handle sewage, sludge, and industrial corrosive wastes in a number of installations, such as sewage lift stations, industrial-waste treatment plants, hotels and office buildings, where the basement is below the gravity sewer line. Pneumatic ejectors are also being increasingly considered for use in pressure-sewer systems where entire areas of a city pump sewage through pressure mains to the treatment plant.
Pneumatic ejector systems require hermetically sealed receivers and hold the raw sewage or corrosive effluent until a predetermined level in the receiver is reached. Compressed air is then automatically admitted to the receiver and the sewage or corrosive liquid is then discharged through check and sometimes gate valves to the pressure mains. Liquid-level sensors must be placed in the hermetically sealed receivers, and the signal transmitted to an electric motor, a diesel or gasoline engine which drives a pneumatic compressor. Electrical or metal mechanical sensors sealed within the container may be subject to extremely corrosive liquids and gases.
Electrodes or electrical contacts are soon grease-coated or fouled and require frequent servicing. Mechanical sensors, which are usually float-actuated, become corroded, or clogged by the materials frequently present in sewage or industrial wastes. Such conditions lead to system unreliability, access difficulty and prohibitive maintenance costs.
If the receiver is filled with flammable liquid or explosive gases, electrically operated liquid level sensors are either prohibited or extremely expensive because of the need to render them explosion proof.